Thermal analyses of the W7-X plasma vessel for operation phase 2

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Abstract

The W7-X plasma vessel (PV) is part of the cryostat wall and forms a vacuum and thermal barrier between the hot vessel interior and the superconducting coils. The vessel is thermally loaded from the inside and is actively cooled by numerous cooling water pipes in order to control its temperature.

However, due to the complex geometry and large number of different types of ports, the cooling pipes are distributed irregularly with varying distances in between them. So the temperature distribution of the PV becomes quite inhomogeneous even with homogeneous load density, and impermissible temperature hot spots >130 °C may arise. Particularly for the upcoming long pulse operation phase 2 (OP2), it is necessary to know the locally allowed thermal loads on the PV. Therefore, temperature distribution maps for different heat loads from 1 kW/m2 to 12 kW/m2 were calculated in order to determine the corresponding hot spots.

The available CAD geometry was not convenient for this extensive and complex analysis; therefore, a simplified shell-beam model was employed to get an approximate temperature distribution, and a correction formula was derived to calculate more accurate values at the positions of interest.

In some areas with large distances between cooling pipes, additional copper stripes are welded onto the PV to improve the heat transfer. For these regions local models were built to calculate the temperature distributions, and, in the course of this analysis, to judge the effect of the copper stripes.

The calculation results indicate positions to be additionally protected and will be used to determine safe operation limits.

Introduction

First plasma operation of the stellarator Wendelstein 7-X (W7-X) started at the end of 2015. During the two-year-long operation phase 1 (OP1), short pulses were carried out with an energy limit of 200 MJ. In the upcoming OP2, the pulse length shall reach 30 min and the total energy 18 GJ [1]. The W7-X plasma vessel (PV) is part of the cryostat wall and forms a vacuum and thermal barrier between the hot vessel interior and the superconducting coils, the latter are protected against thermal radiation by a cryogenic shield and multilayer insulation. This insulation system consequently covers also the outside of the PV. The vessel is loaded from the inside mainly from thermal and ECRH stray radiation, some conduction from the backside of the in-vessel components, and remaining plasma radiation from gaps between the latter. The PV is actively cooled by numerous cooling water pipes (Fig. 1) in order to control its temperature. For the long pulse OP2 it is necessary to know the locally allowed thermal loads on the PV and consequently also on the cryostat thermal insulation. Therefore, a numerical analysis was performed in order to determine the allowed local internal heat loads considering the varying distances between the cooling pipes.

Section snippets

Hot spot calculation

The PV has a helically twisted shell geometry with mean major and minor torus diameters of 12 m and 8 m, respectively, and a wall thickness of 17 mm [2]. Due to this shape with its convexes and concaves, and the numerous port openings (in total 299), the cooling pipes are distributed irregularly. Because of this and the low thermal conductivity stainless steel (1.4429 equivalent to 316 L N) wall material, hot spots over 130 °C may arise which are not permissible regarding the heat load on the

Cu stripe reinforcement

In order to reduce the worst temperature hot spots at positions between cooling pipes which are far apart from each other, Cu stripes were welded onto the PV outer surface to increase the local heat conduction.

In Fig. 9, a typical hot spot without Cu stripe is demonstrated on the left which is located at the center of a narrow uncooled area between two port holes, far away from the cooling pipes on both sides. On the right of this figure, the attached 2 mm thick Cu stripe is shown. There are in

Conclusions

During the upcoming W7-X long pulse operation phase 2 the plasma will be operated with much more energy than in OP1, and it is necessary to determine the allowable local heat loads onto the PV to ensure safe operation of the machine.

The first step is to find out possible hot spots. A simplified shell model of the PV was analyzed under uniform heat loads ranging from 1 kW/m2 to 12 kW/m2, and the regions with temperatures above the allowed 130 °C can be located. However, the temperature from the

Declaration of Competing Interest

The authors report no declarations of interest.

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Author list: T. Klinger et al., Nuclear Fusion 59 (2019) 112004 doi:10.1088/1741-4326/ab03a7.

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